Thermosetting resins based on ethylenically unsaturated N,N-bis(imides) ##STR1## wherein U represents a divalent radical containing a carbon-to-carbon double bond and A represents a divalent radical having at least two carbon atoms, are well known in the art. These bis(imides) may be converted to polyimide resins by heating as taught by Grundschober and Sambeth, U.S. Pat. No. 3,380,964. Alternately, they may be coupled with appropriate multifunctional reactants such as aromatic diamines--see for example, Bargain and Combet, U.S. Pat. No. Re. 29,316. Inasmuch as the rate of polymerization of the bis(imides) can be controlled by temperature, it is customary to carry out a partial polymerization to form an oligomer or "prepolymer" which still retains some degree of solubility in certain solvents. The prepolymer solutions are then used to impregnate fibrous materials such as graphite or glass cloth. These impregnated mixtures are conventionally referred to as "pre-pregs". The solvent is then stripped from the pre-preg, and further heat and usually pressure are applied to complete the polymerization of the bis(imide) and form the final cured composite article with the fibers held in a resin matrix. Cured bis(imide) resins and composites made therefrom have excellent chemical resistance and good thermal stability.
Recently, Wank and Harper in U.S. Pat. No. 4,732,963 have disclosed a new family of thermosetting resins based on the bis(isoimide) structure: ##STR2## wherein U represents again a divalent radical containing a carbon-to-carbon double bond and A represents a divalent radical having at least two carbon atoms. These bis(isoimides), in combination with dihydric phenols, can be partially polymerized to form prepolymers which are readily soluble and stable in low-boiling solvents such as acetone and methyl ethyl ketone. Conventional bis(imide) prepolymers, on the other hand, must be dissolved in high boiling solvents such as dimethylformamide or N-methylpyrrolidone, which are more difficult to remove during subsequent processing--moreover these solutions are relatively unstable. Once finally cured, however, bis(isoimide) resins possess the characteristic advantages of the polyimides--excellent chemical resistance and thermal stability. The same raw materials can be used to prepare both conventional bis(imides) and the bis(isoimides) of Wank and Harper. An unsaturated carboxylic acid anhydride such as maleic anhydride is reacted with a diamine, preferably an aromatic diamine such as methylene dianiline to form a bis(maleamic acid) EQU U(COOH)--CO--NH--A--NH--CO--U(COOH)
wherein U and A are defined as hereinabove. Dehydration of the bis(maleamic acid) with acetic anhydride-sodium acetate yields a conventional bis(imide). On the other hand, dehydration with dicyclohexylcarbodiimide will yield the bis(isoimide)--see Sauers, Cotter, and Whelan, Journal of Organic Chemistry vol. 26(1), p. 10 (1961).
We have now discovered that unexpected improvements in the thermal properties of the bis(isoimide) resin may be achieved by replacing a small percentage of the unsaturated carboxylic acid anhydride starting material with a stoichiometrically equivalent amount of a poly(diolefin)-unsaturated carboxylic anhydride adduct. Reaction with a diamine, followed by dehydration of the bis(maleamic acid) product with dicyclohexylcarbodiimide, yields a bis(isoimide) mixture incorporating the poly(diolefin) functionality. This bis(isoimide) mixture, like the bis(isoimides) of Wank and Harper, can be polymerized with a dihydric phenol to yield prepolymers with solubility in low boiling solvents. These prepolymers can be further cured to form final resins with glass transition temperatures significantly higher than those of the bis(isomide)-dihydric phenol resins disclosed by Wank and Harper.